Lesson Notes By Weeks and Term v4 - SHS 3
PERIODICITY
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
PERIODICITY
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
Subject: Chemistry
Class: SHS 3
Term: 2nd Term
Week: 6
Grade code: 1.2.1.LI.2
Strand code: 2
Sub-strand code: 1
Content standard code: 1.2.1.CS.1
Indicator code: 1.2.1.LI.2
Theme: SYSTEMATIC CHEMISTRY OF THE ELEMENTS
Subtheme: PERIODICITY
This page supports the lesson note with a companion video and a short classroom-ready summary.
For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.
Welcome, future scientists and engineers! Today, we explore one of the most powerful tools in chemistry: the Periodic Table. Think of it not just as a chart of elements, but as a map that reveals predictable patterns in their behaviour. Understanding Periodicity—the repeating trends of properties—allows us to predict how elements will react, what kind of materials they will form, and why they are useful. In Ghana, from understanding the minerals we mine like gold (Au) and bauxite (Al), to knowing which fertilisers (containing N, P, K) are best for our cocoa farms, the principles of periodicity are at work all around us. This lesson will unlock the "why" behind these patterns.
A. The Periodic Law
The foundation of our topic is the Periodic Law. Statement: The physical and chemical properties of the elements are a periodic function of their atomic numbers. Explanation: This means that if you arrange the elements in order of increasing atomic number (the number of protons), you will see a regular, repeating pattern of properties. For example, Lithium (Li), Sodium (Na), and Potassium (K) are all highly reactive metals that behave similarly. This is because their properties repeat at regular intervals, or "periods". B. The Underlying Factors: A Tug-of-War
Almost all periodic trends can be explained by the interplay between two opposing forces within an atom: Effective Nuclear Charge (Zeff): This is the net positive charge experienced by an outer-shell (valence) electron. It's like the "pulling power" of the nucleus. Across a Period: Zeff *increases*. Why? Protons are being added to the nucleus, but electrons are being added to the *same* principal energy level (shell). The increased nuclear charge pulls the electrons in more tightly. Down a Group: Zeff remains relatively *constant* for valence electrons. Although the nuclear charge increases, the number of inner-shell electrons also increases, creating a shielding effect. Shielding Effect (or Screening Effect): The inner-shell electrons repel the outer-shell electrons, "shielding" or "screening" them from the full attractive force of the nucleus. This is directly related to the principal quantum number (n), which represents the energy level or shell. Across a Period: Shielding is relatively *constant* because no new shells are being added. Down a Group: Shielding *increases significantly*. Why? A new principal energy level (n=2, n=3, etc.) is added for each new period. These additional inner shells are very effective at blocking the nuclear pull from reaching the outermost electrons.
Summary of Forces: Across a Period: Zeff wins. The pull gets stronger. Down a Group: Shielding wins. The pull on outer electrons gets weaker. C. The Periodic Properties